A large number of tumors and cell lines of BALB/c and NFS/N strains of mice transformed by Rauscher and Moloney murine leukemia viruses (R-MuLV and M-MuLV) were established in continuous culture and tested for the presence of Fc receptors, as well as intracellular and surface IgM molecules. The preliminary results indicated that these cell lines belong to different stages of B cell development. R-MuLV appears either to induce transformation of a stem cell that undergoes further differentiation and becomes blocked at various stages of development, or transforms B-cells at various stages of differentiation. Further analysis of the nature of immunoglobulin gene rearrangements suggested that these cell lines belong to five successive stages of B cell development or B-cell lineages. Since the c-myc gene is observed to play a role in plasmacytomagenesis, R-MuLV-induced cell lines were studied for c-myc rearrangement. The 5' Eco RI and Hind III sites were rearranged in cells belonging to a particular developmental lineage. In T cells transformed by M-MuLV, the immunoglobulin gene rearrangement is mainly restricted to the heavy chains. The c-myc rearrangement was rare in these cell lines.